Field of the Invention
The invention relates to a magnetic-inductive flowmeter and a method for operating a magnetic-inductive flowmeter.
Description of Related Art
Thus, the subject-matter of the invention is initially a magnetic-inductive flowmeter, having at least one measuring tube for the flow of an electrically conductive medium, having at least one magnetic field generator for generating at least one changing magnetic field running at least partially perpendicular to the longitudinal axis of the measuring tube, having at least two measuring electrodes—in particular, contacting the medium—, having a signal voltage source or signal current source connected to the measuring electrodes for generating conductivity measuring signals, having a control circuit for the magnetic field generator and for the signal voltage source or the signal current source as well as having an evaluation circuit.
Magnetic-inductive flowmeters have been known extensively in the prior art for decades. As an example, reference is made here to the citation “Technische Durchflussmessung” by professor Dr.-Ing. K. W. Bonfig, 3rd Edition, Vulkan-Verlag, Essen, pages 123 to 167 and to the citation “Grundlagen Magnetisch-Induktive Durchflussmessung” by Dipl.-Ing. Friedrich Hoffmann, 3rd Edition, 2003, publication of the company KROHNE Messtechnik GmbH & Co. K G.
The basic principle of a magnetic-inductive flowmeter for flow measurement of a flowing medium is traced back to Michael Faraday, who proposed, in 1832, the use of the principle of electromagnetic induction for measuring the flow velocity of an electrically conductive medium.
According to Faraday's law of induction, an electric field strength is formed perpendicular to the direction of flow of the medium and perpendicular to the magnetic field in a flowing, electrically conductive medium interfused by a magnetic field. Faraday's law of induction is thus exploited in magnetic-inductive flowmeters in that a magnetic field fluctuating over time during the measurement process is generated by means of a magnetic field generator having at least one magnetic field coil, normally two magnetic field coils, and the magnetic field at least partially interfuses the electrically conductive medium flowing through the measuring tube. Here, the generated magnetic field has at least one component perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
As mentioned above, if the magnetic-inductive flowmeter being described here has at least one magnetic field generator “for generating a magnetic field running perpendicular to the longitudinal axis of the measuring tube”, then it is mentioned here that the magnetic field preferably runs perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium; however, it is sufficient when a component of the magnetic field runs perpendicular to the longitudinal axis of the measuring tube or perpendicular to the direction of flow of the medium.
It is also described above that the magnetic-inductive flowmeter described here also has at least two measuring electrodes—in particular, contacting the medium. These measuring electrodes are used for tapping a measuring voltage induced in a flowing medium. Preferably, the virtual connection line of the two measuring electrodes runs essentially perpendicular to the direction of the magnetic field interfusing the measuring tube perpendicular to the longitudinal axis of the measuring tube. In particular, the measuring electrodes can be provided in such a manner that their virtual connection line actually runs—more or less—perpendicular to the direction of the magnetic field interfusing the measuring tube.
Finally, it is described above that the measuring electrodes are, in particular, such that they come into contact with the medium. Indeed, of course, the electric field strength generated by induction in the flowing, electrically conductive medium can be tapped by direct, i.e., galvanic measuring electrodes in contact with the medium as a measuring voltage. However, there are magnetic-inductive flowmeters in which the measuring voltage is not tapped by direct, i.e., non-galvanic, measuring electrodes in contact with the medium, rather the measuring voltage is capacitively determined.
The primary use of magnetic-inductive flowmeters of the type described here is naturally the measurement of the flow of a medium through a measuring tube, namely a medium that has at least a low electric conductivity, i.e., flow measurement. However, the use of magnetic-inductive flowmeters is not limited to this use. In particular, magnetic-inductive flowmeters can also be used particularly for conductivity measurement.
Magnetic-inductive flowmeters can be used for conductivity measurement when the conductivity of the medium whose flow is to be measured, for whatever reason, is of interest or importance. In particular, however, the conductivity of the medium whose flow is to be measured, is of importance for flow measurement itself because the measuring voltage tappable at the measuring electrodes is not only dependent on the magnetic field strength of the magnetic field generated by the magnetic field generator and the flow to be measured, but moreover also on the conductivity of the medium whose flow is to be measured.
The magnetic-inductive flowmeter described above is such that it is used and suitable not only for flow measurement, but also for conductivity measurement, namely,—for conductivity measurement—it has a signal voltage source or a signal current source with which conductivity signals are generated.
Magnetic-inductive flowmeters of the type described above, which are also used and suitable for conductivity measurement, are known, for example, from European patent 0 704 682, German Patent DE 692 32 633 C2, as well as from German Application DE 102 43 748 A1 and corresponding U.S. Pat. No. 6,804,613 B2 and German Application 10 208 258 A1.